The microwave electron gun, first described in U.S. Pat. No. 4,641,103, has proven to be a highly effective source of electrons for applications requiring high peak current and high beam quality such as free electron lasers and accelerators for particle physics research. Broadly, such a gun subjects the electrons emitted from a cathode to an intense microwave electric field for acceleration, and then typically blocks all but a narrow range of momentum to provide the bunching required by the linear accelerator. The gun comprises a resonant microwave cavity and a cathode mounted in the cavity wall.
The resonant microwave cavity, when supplied with microwave power, supports an electromagnetic field having a high-gradient electric component directed along an acceleration axis. The cavity is formed with an exit aperture at a location relative to the cathode such that emitted electrons are accelerated along the axis and pass through the exit aperture. Bunching, if required, is provided by a momentum analyzer system, which may include a dispersive magnet and a slit. An electron emerging from the cavity has an energy (energy and momentum have a one-to-one relationship, and thus will sometimes be used interchangeably) determined by the phase of the microwave field at the time of that electron's emission. The magnet causes electrons with different energies to follow different trajectories, while the slit is disposed to block those electrons having energies outside a desired narrow range of energies and phases. Thus, only those electrons having energies corresponding to a narrow range of phases are permitted to pass through the momentum analyzer, thereby forming a pre-bunched electron beam for injection into a linear accelerator.
However, use of the technology has been complicated by the back-heating phenomenon, in which electrons emitted from the cathode late in the accelerating phase of the applied microwave field are decelerated by the field before they escape the cavity, and are returned to the cathode with sufficient energy to raise the cathode temperature (and hence the emitted current density) as time progresses during the pulse. While the phenomenon has little impact on operation for modest emitted currents and short RF pulses, the temperature rise for higher cathode currents and/or longer pulses can substantially alter the beam current during the pulse, causing the energy of the electrons leaving the cavity to droop due to beam loading. In the worst case, this can lead to thermal runaway in which the cathode temperature rises uncontrollably due to ever-increasing back-heating. These electrons are referred to as backstreaming electrons.
Efforts to eliminate back-heating have included application of a transverse magnetic field to deflect the backstreaming electrons so that they strike the walls of the cavity surrounding the cathode instead of the cathode, and optimization of the dimensions and configuration of the cavity to reduce the chances that the electrons emitted late in the accelerating phase of the field will be returned to the cathode. An attempt has also been made to use ring-shaped or toroidal cathodes to exploit the tendency of the back-heating electrons in these designs to return to the cathode near the axis where they would strike a non-emissive component of the cathode assembly. None of these approaches has succeeded in reducing the temperature rise of the cathode to the level in which cathode emission remains substantially constant during the pulse.